In recent years, as the recording information amount has been increasing in the information technology field, large capacity optical discs have been demanded. The capacity of an optical disc i.e. the density of recording information can be increased by increasing the numerical aperture (NA) of an objective lens, and using short wavelength laser light to be emitted from a light source.
A light source for emitting light of 660 nm wavelength and an objective lens of 0.6 in NA are used for currently available and widely used digital versatile discs (DVD).
On the other hand, the density of recording information as next-generation optical discs is increased by using a blue laser for emitting light of 405 nm wavelength for blue ray discs (BD) and HD-DVD and setting NA to 0.85 in BD, and 0.65 in HD-DVD. High-performance, miniaturization, and cost reduction are required for an optical head to be used in an optical information recording and reproducing device for optically recording or reproducing information with respect to an optical disc using the blue laser.
FIG. 32 is a conceptual diagram showing an arrangement of an optical head 76 for use in a conventional and general optical information recording and reproducing device. Referring to FIG. 32, the reference numeral 7′ indicates an optical disc, 71 indicates a semiconductor laser for emitting laser light of 660 nm wavelength, 72 indicates an objective lens for condensing light to be emitted from the semiconductor laser 71 into a small light spot to be incident onto the optical disc 7′, 73 indicates an objective lens actuator, as condensing element driving means, for driving the objective lens 72 to correct plane displacement or axial displacement of the optical disc 7′, 74 indicates a half mirror made of a glass material and for splitting reflection light from the optical disc 7′, 75 indicates a light receiving element for receiving reflection light from the optical disc 7′, and 78 indicates a detection lens made of a glass material and for condensing reflection light from the optical disc 7′ onto the light receiving element 75.
The optical head 76 is constituted of the semiconductor laser 71, the objective lens 72, the objective lens actuator 73, the half mirror 74, the detection lens 78, and the light receiving element 75. The light receiving element 75 functions as an RF signal light receiving element for detecting an information signal from the optical disc 7′, and a servo signal light receiving element for detecting a servo signal from the optical disc 7′.
In the conventional optical head 76, for instance, a multi-divided light receiving element is used as a light receiving element for detecting an RF signal and a servo signal. A light receiving element to be used in a conventional and general optical head is described referring to the diagrams shown in FIGS. 33A and 33B.
FIG. 33A is a plan view of a 2-divided photodiode 80 to be used as a light receiving element, and FIG. 33B is a cross-sectional view taken along the line D-D′ in FIG. 33A.
Referring to FIGS. 33A and 33B, the reference numeral 80 indicates a 2-divided photodiode. 21a and 21b each indicates a light receiving cell made of a semiconductor material and having a size of e.g. 0.75 mm×0.75 mm in area and 0.3 mm in thickness. 28 indicates a dividing line for separating the light receiving cell 21a and the light receiving cell 21b. 22a and 22b indicate upper electrodes to be formed on the surfaces of the light receiving cells 21a and 21b, respectively. 23a, 23b, 23c, and 23d each indicates a frame terminal. 25 indicates a lower electrode continuously formed on the backside surfaces of the light receiving cells 21a and 21b. 26a indicates a metal wire for electrically connecting the upper electrode 22a and the frame terminal 23a 26b indicates a metal wire for electrically connecting the upper electrode 22b and the frame terminal 23b. 87 indicates a resin protector which is made of a cured transparent epoxy resin, is adapted for protecting the 2-divided photodiode 80, with a size of e.g. 4 mm×5 mm in area and 1.9 mm in thickness, and is adapted for defining the outer shape of the 2-divided photodiode 80. The frame terminal 23c and the frame terminal 23d are integrally formed into a frame member 24.
The lower electrode 25 formed on the backside surfaces of the light receiving cells 21a and 21b is electrically connected to the frame member 24. The upper electrodes 22a and 22b are electrically connected to the frame terminals 23a and 23b by the metal wires 26a and 26b, respectively. In the 2-divided photodiode 80, the frame terminal 23a, 23b is an anode terminal, and the frame terminal 23c, 23d is a cathode terminal. Light received on each of the light receiving cells 21a and 21b is converted into a photocurrent. The photocurrent is outputted from the frame terminal 23a, 23b. A portion constituted of the light receiving cells 21a and 21b, and the upper electrodes 22a and 22b is also called as a light receiving chip 29.
The 2-divided photodiode 80 to be used as a light receiving element in a conventional and general optical head has the following problem.
Generally, an epoxy resin is used as a material of a resin protector for protecting a 2-divided photodiode to be used in an optical head in an optical information recording and reproducing device for an optical disc such as DVD, using laser light of 660 nm wavelength. However, in the case where an epoxy resin is used as a material of a resin protector for protecting a 2-divided photodiode in an optical information recording and reproducing device for next-generation discs such as BD or HD-DVD, using a blue laser for emitting light of 405 nm wavelength or a like device, the epoxy resin may be oxidized by UV absorption in receiving blue laser light of 405 nm wavelength. As a result, the color of the resin protector may be changed by formation of colored substance, and the light transmittance of the resin protector may be gradually reduced. As the light transmittance of the resin protector is reduced, the light receiving amount on the light receiving cells 21a and 21b may be reduced. As a result, detection of light indicating an RF signal or a servo signal may be insufficient.
In order to solve the above problem, for instance, patent document 1 discloses a 2-divided photodiode 162 having an arrangement as shown in FIGS. 34A and 34B, wherein a glass plate is fixedly disposed on an optical path of light in a light receiving element to block light transmittance through an epoxy resin. FIG. 34A is a plan view of the 2-divided photodiode 162, and FIG. 34B is a cross-sectional view taken along the line E-E′ in FIG. 34A.
Referring to FIGS. 34A and 34B, the reference numeral 161 indicates a glass plate, 87 indicates a resin protector, 21a and 21b each indicates a light receiving cell, 28 indicates a dividing line, 22a and 22b each indicates an upper electrode, 23a, 23b, 23c, and 23d each indicates a frame terminal, 25 indicates a lower electrode, 26a and 26b each indicates a metal wire, 160 indicates a light receiving chip constituted of the light receiving cells 21a and 21b, and the upper electrodes 22a and 22b. The frame terminal 23c and the frame terminal 23d are integrally formed into a frame member 24.
The 2-divided photodiode 162 is different from the 2-divided photodiode 80 in the point that the glass plate 161 disposed on the optical path of light in the light receiving element is fixed to the resin protector 87 made of an epoxy resin and having an outer shape with a hollow portion and a step portion.
The 2-divided photodiode 162 has a size of e.g. 5 mm×6 mm in area, and 1.9 mm in thickness.
In the 2-divided photodiode 162, solely semiconductor laser light of 405 nm wavelength is transmitted through the glass plate 161, and reaches on the light receiving cells 21a and 21b, without transmitting through the resin protector 87 made of an epoxy resin. Accordingly, in the 2-divided photodiode 162, there is no likelihood that the light receiving amount of the light receiving chip 160 may be reduced resulting from a reduced light transmittance.
However, the 2-divided photodiode 162 has a problem that the production process may be complicated due to an increased number of production steps by an increased number of parts, or the outer dimensions of the 2-divided photodiode 162 may be increased.
A detection lens, an objective lens, or a like element to be used in a conventional and general optical head is made of a glass material, as recited in e.g. patent document 2. A detection lens made of a glass material is free from a problem that the transmittance may be lowered. However, in the case where a detection lens, an objective lens, or a like element is made of a glass material, the production process may be complicated. Also, there is a demand for a lens made of a transparent resin, which is easily fabricated by injection molding, to reduce the size of a detection lens for miniaturization of an optical head, and to product an axially asymmetrical lens.
Patent document 1: JP No. Hei 7-73503-A
Patent document 2: JP No. Hei 1-287830-A